1. The Field of the Invention
The present invention relates generally to implants for use in orthopedic surgery, and more particularly, but not necessarily entirely, to molds for forming temporary orthopedic implants in the knee.
2. Description of Related Art
The anatomy of the knee is primarily composed of two major bones, the tibia and the femur. These two bones meet to form a hinge joint between the proximal tibia and the distal femur. The knee joint is protected by the patella, or the kneecap. The knee joint is cushioned by articular cartilage that covers the ends of the tibia and femur, as well as the underside of the patella. The lateral meniscus and medial meniscus are pads of cartilage that further cushion the joint, acting as shock absorbers between the bones.
A healthy knee joint is able to withstand great forces that are exerted as the knee flexes and extends and supports the weight of the body. However, when the knee joint becomes diseased, damaged or is otherwise unable to withstand the forces required of that joint, it may become necessary to reconstruct or replace the knee joint. When replacement is necessary, the natural knee joint is replaced with a prosthetic knee joint. A typical prosthesis includes a femoral component and a tibial component. During a replacement knee surgery, typically portions of both the tibia and femur must be resected to allow the placement of prosthetic tibial and femoral components, which are anchored to the respective bones.
Sometimes, a small percentage of patients who undergo a total knee replacement surgery suffer from infections in the knee joint at the surgical site. To alleviate the effects of the infection, a two-stage revision of the failed knee replacement is necessary. The two-stage revision includes the following procedures. First, the prosthesis must be surgically removed and the site debrided and cleansed extensively in order to rid the site of the infection. Before a new, permanent prosthesis can be placed in the old surgical site, the site must be free of infection. It is typical for a temporary antibiotic-impregnated cement spacer to be used as part of the therapy to rid the site of infection. Disinfecting the site of infection may take between 6-8 weeks and 3-4 months in most circumstances. Therefore, it is common for surgeons to replace the old prosthetic knee with a temporary implant, typically made of bone cement, during the 6-8 week period while the infection is cleared up and before the new prosthesis is surgically implanted. The second and final step requires a separate revision surgery to then replace the temporary implant with a permanent prosthetic implant.
In the past, surgeons have been left to their own devices when forming cement implants, including the use of negative molds. The process of making a negative mold consists of the surgeon creating a mold by inserting a portion of bone cement into a bowl or other mixing container and allowing the cement to nearly cure. Prior to complete curing of the bone cement, the surgeon inserts the articulating end of the femoral component into the bone cement to create a mold. Using that mold, the surgeon then applies an oil to the mold creating a barrier for separating the cement implant from the cement mold. After applying the oil, cement may be poured into the mold allowing it cure, after which the surgeon attaches the resulting bone cement implant onto the femur as a temporary replacement. Negative molds may be used for both femoral and tibial components alike.
Other methods used in the past of forming temporary implants include surgeons creating the implant with their own hands or simply putting a block of cement between the tibia and the femur to act as a spacer. However, there are many problems associated with such methods and designs, namely increased surgical time due to the preparation and formation time needed for creating the implant. Particular problems associated with the block or spacer method include completely immobilizing the knee in an extended position, after surgery, for the entire 6-8 week period, which in turn leads to soft tissue damage and further complicates the revision surgery. Therefore, reproducing the knee joint using temporary implants that simulate the natural tibial and femoral components of the knee joint is much more desirable because it permits the patient to move his/her leg through a minimal range of motion. The range of motion, while limited, significantly increases the patient's comfort over the 6-8 week period allowing the patient to bend his/her knee for sitting in a chair or for riding in a car and also increases the ease of the revision surgery because the soft tissue has not been damaged to the same extent as when the knee is completely immobilized.
Attempts have been made in the prior art to provide alternatives to surgeons creating their own negative molds or even molding a temporary implant by hand. For example, U.S. Pat. No. 6,155,812 (granted Dec. 5, 2000 to Smith et al.) discloses a cement mold for use in forming a temporary orthopedic implant used in an orthopedic surgical procedure, said mold including a first and second mold joined together by a coupling mechanism. This device is characterized by several disadvantages, including the need for many different sized molds to accommodate the differences in size of patients tibias and femurs, the need for multiple pieces that must be joined together and the need to use the mold outside the patient's knee joint such that the implant must first be formed and then attached to the tibia or femur in two separate steps.
There are several other temporary bone cement molds known in the prior art, such as that disclosed in an article entitled “Temporary Articulating Methylmethacrylate Antibiotic Spacer (TAMMAS)—A New Method of Intraoperative Manufacturing of a Custom-Articulating Spacer” found in The Journal of Bone and Joint Surgery 83:S92-97 (2001). This reference discloses a method of conforming sterilized, heavy aluminum foil to the distal portion of the femur or the proximal portion of the tibia such that an aluminum mold is created. Bone cement is later poured into the mold and the cement is formed into the shape of the femoral or tibial component. However, this technique is disadvantageous because it requires the surgeon to conform the foil to the bone and to further form the bone cement into the shape of a femoral or tibial component. In this setting, another disadvantage in the aluminum foil is its ductility. The foil is unable to spring back into its original shape or position making it very difficult to work with and to maintain its shape as the cement is poured into the mold and while the cement is allowed to cure.
Other attempts have been made to provide a mold that is capable of maintaining its original shape. However, such attempts have proven to be unsuccessful because the rigid, non-resilient nature of the materials used, such as metal or a metal alloy. The rigid, non-resilient nature of a metal mold allows the mold to be used repeatedly. However, there are several disadvantages associated with non-resilient molds, for example, non-resilient molds do not have the requisite flexibility to be used in situ, as it is difficult to remove the implant from the mold and there is no flexibility for the mold to be conformed to various anatomical differences in the bones.
It is noteworthy that none of the prior art, known to applicants, provides an elastic mold capable of being attached in situ that further provides for a one piece mold capable of attaching to various sized bone components. There is a long felt need, illustrated by the statistics of increased surgical success of the revision surgery when a patient has a limited range of motion in the knee joint, for an elastic mold for in situ surgical use that allows the bone cement replacement implant to conform to the shape of the knee bones.
The prior art is thus characterized by several disadvantages that are addressed by the present invention. The present invention minimizes, and in some aspects eliminates, the above-mentioned failures, and other problems, by utilizing the methods and structural features described herein.
The features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by the practice of the invention without undue experimentation. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims.